Synthetic aperture radar focusing

ABSTRACT

An extended depth-of-focus synthetic aperture radar (SAR) system (13) mounted on a moving platform, including a controller (120), pulse timer (83), synthesizer (105) and modulator (17) for varying the pulse rate interval (PRI) and/or the radar carrier frequency of radar pulses produced, in order to establish a radar return which, when conventionally processed, results in a SAR terrain map exhibiting extended depth-of-focus under conditions of platform acceleration. Depth of focus is established by ensuring the establishment of two or three separate, independently selected focal points in a target region of interest.

CROSS REFERENCE TO RELATED APPLICATIONS

Three separate commonly-owned U.S. Patent applications are filed on evendate herewith relating to the same general subject matter and having theindicated respective U.S. Patent Application Ser. Nos. 842,961, 842,959and 842,951, the first and third application are now U.S. Pat. Nos.4,706,088 and 4,706,089 respectively, entitled SYNTHETIC APERTURE RADARFOCUSING.

TECHNICAL FIELD

This invention is directed toward the technology of motion-compensated,synthetic aperture radar (SAR) systems.

BACKGROUND ART

Synthetic aperture radar (SAR) systems in general are well known.Further, motion-compensated synthetic aperture radar systems have beenin development and use for many years.

One object of such systems is to generate terrain maps from a movingplatform in flight. The map generated thus describes a selected portionof the terrain observed by an aircraft for example.

The radar system can be adjusted by the pilot as desired, within certainbounds of course, to determine the precise location and size of thepatch or portion of terrain being observed, mapped or monitored.Generally, the terrain to be mapped is below the aircraft and off to theside. The aircraft typically flies at a given altitude and variablevelocity, and its radar has a downward angle of view.

The radar transmits repeated radio frequency (RF) pulses at a selectedpulse repetition frequency (PRF) or its inverse, the pulse repetitioninterval (PRI), toward the terrain to be mapped. After each pulsetransmission, the radar receiver waits to receive any return from theselected terrain.

Since the radar has been set to receive a return from a given,preselected general region--the selected map area--and the radar signalstravel at a known velocity, it is known precisely when a return islikely to come from terrain at a given range. In fact, the expected timeof return of signals from subdivisions within the area to be mapped canalso be precisely determined.

Accordingly, for each transmitted pulse, depending upon the terrain,there will be return corresponding to a preselected number of rangeportions of the area to be mapped. It is of course not possibleinitially to resolve precisely from which angular location the returncomes. Such resolution is however possible after the data for each rangebin has been spectrally analyzed (as by Fourier transform for example).

Nonetheless, meaningful information is acquired in the form of a complexnumber representative of the return phase and magnitude for apredetermined number of range bins in the selected terrain area. Thosecomplex numbers, representing the return over time derived from a singleoutput pulse, are representative of all of the range bins covering theselected terrain.

As the radar platform continues flight, further pulses are transmittedand their return is received. For each subsequent transmitted pulse, acomplex return is received, and this in turn is stored in acorresponding range bin. In the end, a complex matrix will have beenestablished, which contains bundles of information regarding the terrainat selected ranges for the map region selected.

As suggested above, in order to map the terrain, these bundles ofinformation need to be analyzed as to angle or location. This is done byFourier transforms, which convert the information into frequencyinformation, thereby permitting the angle and amplitude of the return tobe established. There will be a separate Fourier transform performed foreach range bin.

To correct for the changes in position of the radar platform overtime--that is to correct for the change in position between successivepulse transmissions, which affects the phase of the return--it isnecessary to perform a function on the return signals known as focusing,or motion compensation. This process causes a single point on the map,known as map reference center, to come into sharp focus.

This is done conventionally by placing accelerometers in the movingradar platform. These accelerometers provide signal indications of thechanges in motion during spatial translation of the platform. In otherwords, acceleration information is produced. This does not, of course,directly specify the new location to which the platform has traveled.

To accomplish this, two integrations of the acceleration informationare, in effect, constructed. First, the acceleration components in eachof the standard coordinate directions are integrated to providecorresponding velocity information. Then another integration isperformed upon the velocity information to produce actual locationindications with respect to the standard coordinate directions.

From this new position information, a phase correction can be applied tosignals coming from the same map location, which correction correspondsto the change in range. By applying this correction, the effective rangefrom the radar center or platform to the map reference center is forcedto be equal for successive radar pulses. As a result, the return from anobject located at the map reference center will appear in the radar mapas a point in sharp focus.

When all of the information regarding the same terrain portion for asufficient number of successive pulses is analyzed by Fourier transform,the resulting frequency information is translated into phase and fieldangle information. Thus, the intensity of return for a specific maplocation can be determined.

Alternately, the motion information, instead of being computed for apreselected map reference center from accelerometer measurement data,can be extracted from the radar returns of prominent radar scatterers.This method is known as "autofocus".

According to the prior art, producing radar pulses at a regular pulserate intervals is well known. Moreover, setting the carrier frequency ofthe transmitted pulses to a specific predetermined value is well known.

It is also well known, however, that the geometric extent of the focusedarea of maps produced by current synthetic aperture radar systems isreduced by changes in aircraft movement and flight maneuvers. In otherwords, established SAR mapping techniques are subject to severelimitations in terms of depth of focus, when the radar platform movesabruptly.

Two methods have been used to mitigate the reduction in depth of focusunder accelerated aircraft motion. According to one method, by usingstandard focusing techniques, a multiplicity of SAR maps are produced ofadjacent areas, each having a depth of focus limited by the aircraftacceleration. The size of the resulting map defines the extent of theincrease in the depth of focus.

Such an approach, which uses a multiplicity of map reference centers(or, in the case of "autofocus", of prominent scatterers) is, however,typically expensive in terms of hardware and/or software developmentcosts as well as in processing time. Alternately, a variation of theradar pulse repetition interval (PRI) or its inverse, the pulserepetition frequency (PRF) is used.

The prior art of PRF variation does include use of more than a singlefocal point; the location of a second point cannot, however, beselected, but it is determined by the nature of the particular algorithmfor PRF variation.

DISCLOSURE OF INVENTION

According to the invention, an enhanced depth of focus radar map of aselected region under survelliance is enabled over a widened portion ofthe map surveyed. This is accomplished by varying the pulse repetitionfrequency (PRF) and/or the radar carrier frequency (RF) on the basis ofinformation supplied by aircraft inertial sensors to force one or twoarbitrary points of the map, in addition to a third point such as analready selected map reference point, into sharp focus.

As a result, an improved radar terrain map of predetermined size andlocation can be obtained, which is characterized by an extended depth offocus. This is produced according to the invention by periodicallyadjusting the radar carrier frequency (RF) or the repetition interval(PRI) of each radar pulse, or both, for a radar platform in flight,according to relationships set forth below.

The foregoing and other objects, features and advantages of the presentinvention will become more apparent from the following description ofpreferred embodiments and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a system block diagram of the extended depth-of-focussynthetic aperture radar (SAR) system according to this invention;

FIGS. 2A and 2B are a flowchart illustrating operation of one version ofthe invention in which both carrier frequency and pulse interval arevaried; and

FIG. 3 is a partial flowchart of blocks 500 and 600 which can besubstituted in FIGS. 2A and 2B to show other versions of the inventionin which either carrier frequency or pulse repetition interval alone isvaried.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 shows a block diagram of the synthetic aperture radar (SAR)system 13 according to the invention herein. In particular, the SARsystem 13 includes a modulator 17 which receives a stable radiofrequency carrier signal along line 18 from an exciter synthesizersystem 19.

The modulator 17 effectively modulates a radio frequency (RF) signalreceived on line 18 to produce a series of output pulses on line 20having a selected pulse width. This RF carrier signal "f_(t) " isproduced in exciter synthesizer system 19 at frequency synthesizer 105,according to well-known electrical design techniques using knownhardware elements which receive input frequency "f_(x) " from crystaloscillator 101, and multiply it to establish a higher frequency "f_(t) "to serve as carrier for the transmitted radar signal launched fromantenna 31.

The output pulses on line 20 are amplified by a transmitter power stage27, which feeds a radar antenna 31 via a circulator 34. The transmittedpulses from modulator 17 and passing through circulator 34 thus radiatefrom the antenna 31 to propagate toward a selected target region (notshown) which is under surveillance. A portion of the transmitted signalis reflected from the illuminated target and returned to antenna 31 fordetection as will be shown.

In particular, some of the reflected signal energy is captured uponreturn by the antenna 31 and fed back to the circulator 34 and fromthere to a mixer 39, where it is combined with a suitable differencefrequency 50, i.e. "f_(t) --f_(x) " from the exciter synthesizer system19. This difference frequency, also known as the stable local oscillatorfrequency (STALO), is the transmitted frequency "f_(t) " less thecrystal oscillator frequency "f_(x) " as generated by single sidebandgenerator 106 according to well-known design principles and with the useof conventional hardware. According to one version of the invention, thevalue of "f_(t) " to be combined with "f_(x) " is variable, to accountfor the platform movement, even when the transmitted carrier frequency"f_(t) " is held constant, which would be another version of theinvention.

The output of mixer 39 establishes the reflected signal received byantenna 31 as an intermediate frequency (IF) signal, which is the sameas the crystal oscillator frequency "f_(x) ", because mixer 39 combines"f_(t) " from circulator 34 and the difference "f_(t) -f_(x). The IFsignal is amplified at IF amplifier 55, as shown. The mixer outputsignal is further coherently detected by coherent detector 59 withreference to the phase of a reference signal at the same frequency"f_(x) " on line 61 from the exciter synthesizer system 19 and crystaloscillator 101.

In actuality, there is relative motion between the antenna and theground to be mapped, which results in a doppler shift of the returnfrequency. This very phenomenon makes synthetic aperture radar possible.Thus, the input frequencies to the mixer 39 and to the IF amplifier 55,although labeled "f_(t) " and "f_(x) ", respectively, are in realityoffset from those values by the doppler shift.

The resulting in-phase and quadrature signals respectively "I" and "Q"are then timed, converted from analog to digital form in ananalog-to-digital converter 66, which is subject to a clock signal"f_(c) " established by dividing the crystal oscillator 101 in frequencydivider 102 to produce a lower frequency along timing line 69. Theinformation thus digitized is stored and processed conventionally toestablish a SAR map in processor 77. The output of the processor 77 canbe fed to a map display 79. These particular processes are conventionaland well-known according to the present state of the art of SAR mapping.

FIG. 1 further shows a pulse timer 83 for varying the pulse repetitioninterval (PRI) of the SAR system 13 by controlling the timing of outputpulses from modulator 17, each being at the indicated RF carrierfrequency "f_(t) ".

The timer 83 is shown including two digital registers 85 and 86,respectively, the first being a counter of clock pulses from frequencydivider 102, and the other being a fixed register containing a binarynumber corresponding to the desired time of the next pulse transmission.A comparator 89 in timer 83 triggers the pulse modulator 17, when thenumbers in the two registers are equal. After the transmission of apulse from modulator 17, the counter 85 is reset along lines 85'.

The exciter synthesizer system 19 includes a stable crystal oscillator101, frequency divider 102 effective for establishing a predeterminedclock frequency "f_(c) ", synthesizer 105 including multipliers, addersand phase-lock-loops (PLL) as is well known in the art of designingradar circuits, and single sideband generator 106, as already suggested.

The outputs of the exciter synthesizer system 19 include lines 18 and 61carrying signals charcterized by respective frequencies "f_(t) " and"f_(x) ".

The SAR system 13 further includes a controller 120 which generatessignals that control the operation of the exciter synthesizer 19 in theproduction of transmission frequency "f_(t) ", and other signals whichcontrol pulse timer 83. This generator 120 accordingly determines theprecise time of occurrence and the radio frequency of each transmittedpulse according to the scheme indicated below. Its mechanicalimplementation can take a variety of forms, such as that of dedicatedhardware or a special or general-purpose computer.

The controller 120 employs predetermined initial nominal values forpulse repetition intervals (PRI), i.e. "delta t", which corresponds tothe time between sequential output pulses from modulator 17, and forwavelength "lambda" which corresponds to the operational, or nominaloutput carrier frequency "f_(t) " of the radar system, as well asaccording to one version of the invention, first and second constants,"c₁ " and "c₂ " which are computed on the basis of three preselectedfocal points on the map to be produced as will be shown. These pointsare selected to maintain adequate depth of focus in several regions ofthe area being mapped.

For each radar pulse on line 20 to be produced, new values of "delta t"(or PRI) and/or "delta lambda" are determined as suggested below. Thiswill establish the time at which a next radar pulse is to be transmittedafter the time of a prior pulse transmission, and further establishesthe wavelength of this pulse.

For clarity of illustration "delta t", "delta lambda" and "lambda" arecapitalized in the flowcharts of FIGS. 2A, 2B and 3.

As is well known in the art, frequency and wavelength are functionallyinterrelated in a well known fashion. Most simply stated, in free space,the relationship is f_(t) =c/lambda, where "c" is the speed of light,lambda is the wavelength, and "f_(t) " is the frequency transmitted.

The derivation of the relationships which are central to one version ofthe invention herein, according to which three focal points areestablished in the region mapped and both pulse rate frequency andcarrier frequency are varied, can begin with the following equations,which in turn are based upon principles established even further below:

For i=1,2: a_(i) (delta t)+b_(i) (delta lambda)=c_(i), where thequantities "a_(i) " are equal to the dot products (U_(i) -U₀) [V]; "U₀ "is the unit direction vector to the selected map reference cenmter;"U_(i) " are unit vectors in the direction of each of two other selectedpoints respectively on the map region; and "V" is the velocity vector ofthe radar antenna phase center; "b_(i) " are equal to (r₀-r_(i))/lambda, "r₀ " is the distance from the antenna phase center tothe selected map reference center; and "r_(i) " are the distances fromthe antenna phase center to the selected respective first and secondpoints on the map which are independent from each other and the selectedmap center; and finally, "c₁ " are system constants, as will bediscussed further. All vector multiplication suggested herein is by dotproduct.

The above relationships can be manipulated in accordance with well-knownmatrix and linear algebra techniques to yield the following: "deltat"=(c₁ b₂ -c₂ b₁)/D and "delta lambda"=(c₂ a₁ -c₁ a₂)/D, where "D"=a₁ b₂-b₁ a₂. Accordingly, "delta t"=[c₁ b₂ -c₂ b₁ ]/[a₁ b₂ -b₁ a₂ 9 , andmore particularly: "delta t"=[c₁ (r₀ -r₂)-c₂ (r₀ -r₁)]/[(V)][(U₁ -U₀)(r₀-r₂)-(U₂ -U₀)(r₀ -r₁)], where "r_(i) " and "U_(i) " to are defined forvalues of "i" equal to "0", "1" and "2" in accordance with definitionsalready laid out and presented above.

The constants "c₁ " and "c₂ " are established using initial values for"delta t" and "delta lambda" which are supplied to controller 120 priorto beginning regular operation, and initial values of a₁, a₂, b₁, b₂ bymeans of the following linear equations which have already beendiscussed: c₁ =a₁ (delta t)+b₁ (delta lambda) and c₂ =a₂ (delta t)+b₂(delta lambda); where "a₁ " and "a₂ " are defined by the relationshipa_(i) =(U_(i) -U₀)[(V)] for "i" equal to "1" and "2", and "U_(i) " is aunit vector to selected first and second locations on the region beingmapped and V is the platform velocity; and "b₁ " and "b₂ " are definedby the relationship: b₁ =(r₀ -r₁)/lambda for the same values of "i" asabove, and "r_(i) " are the distances from the platform to saidlocations on the map, and "lambda" of course is a predeterminedinitially selected wavelength used in these linear equations. For thepurpose of evaluating the constants "c₁ " and "c₂ " to be used for thesynthetic aperture, the quantity "delta t" is taken to be the initialpulse repetition interval; "delta lambda" can conveniently be set equalto zero.

Similarly, the relationship for "delta lambda" can be developed furtheras follows: "delta lambda"=[c₂ a₁ -c₁ a₂ ]/[a₁ b₂ -b₁ a₂ ] in turnresulting in: "delta lambda"=[c₂ (U₁ -U₀)V-c₁ (U₂ -U₀)V][lambda]/[(r₀-r₂)(U₁ -U₀)V-(r₀ -r₁) (U₂ -U₀ ]V.

According to other versions of the invention, suggested in FIG. 3, onlytwo focal points are established in the region mapped, and only thepulse rate frequency or the carrier frequency are varied. In thesecases, the formulas for "delta t" and "delta lambda" are as follows:

    "delta t"=c.sub.1 (lambda)/4 pi[V(U.sub.1 -U.sub.0)]

and

    "delta lambda"=[c.sub.1 (lambda)/4 pi-[V(U.sub.1 -U.sub.0)[PRI]][lambda/(r.sub.0 -r.sub.1)];

where "c₁ "=(4 pi)[V](U₁ -U₀)PRI/(lambda); PRI is the initial pulseinterval, which is the inverse of the pulse frequency; "U₀ " is thefirst direction vector at an initial time, "U₁ " is the second directionvector at that time, and "V₀ " is the initial velocity of the radarplatform. In the relationships immediately above, it is assumed thatonly the pulse interval or the carrier frequency (or its relatedwavelength) are varied, and not both of them as defined in therelationships preceding the latter.

The controller 120 accordingly adjusts, in two versions of theinvention, the SAR system 13 to operate at a selected carrier frequency,and more importantly at a mixer reference frequency, (STALO),corresponding to the wavelength "(lambda)_(j) "="(lambda).sub.(j-1)"+"(delta lambda)", as suggested in FIGS. 2A and 2B. Further, in oneof these two versions and in a third version of the invention, a nextpulse 20 to be transmitted at the time t_(j) =t_(j-1) +(delta t), where"j" is the index of the pulse number. This latter expression indicatesthat the time of occurence of a pulse is the time of the former pulseincremented by "delta t", whereby the pulse interval is varied.

Thus, a desired number of returns are coherently detected, stored andprocessed conventionally in processor 77 to yield a SAR map having anenhanced depth-of-focus.

This is set forth generally in the flowchart of FIGS. 2A and 2B, whichshows the basic scheme according to which the invention operates in theversion having both variable pulse interval and variable carrierfrequency. By reference to these Figures, it will also be explained howthe system operates to enhance depth of focus by varing only one ofthese two parameters. In the case of varying both parameter, three focalpoints are established in the region mapped, whereas in the singlevariable parameter cases, two focal points are established.

To proceed, FIG. 2A shows operation beginning with the pilot or operatorof the system selecting the location and size of the terrain region tobe mapped as indicated in block 100. This is typically done for exampleby turning a control knob (not shown) on the actual radar set orconsole. In particular, the operator thereby establishes the aspect,range and size of the region mapped. Blocks 200 and 300 indicate theselection of the focal points defining initial position vectors andranges, "U₀ ", U₁ ", "U₂ ", "r₀ ", "r₁ " and "r₂ " according to apre-established scheme for selection of three focal points. If only twofocal points are to be established, "U₀ ", "U₁ ", "r₀ " and "r₁ " aredefined.

The direction vectors point generally in the same direction and havegenerally the same magnitude, but the slight variations therebetween areof key significance. The distances "r₀ ", "r₁ ", and "r₂ " are therespective distances from the radar antenna to points 0, 1 and 2. In anycase, these vectors and distances correspond to the selection of focalpoints in the map region selected.

Since it is desired that optimally the entire map region be in focus,having the focal points distributed apart in the region is preferred,rather than having the focal points close together. According to oneversion of the invention, assuming the map region to be generallysquare, one of the selected points might be a map reference point at thecenter of the region (as per block 200), and the other or others of theselected points might be corner points of the region, as suggested inpart at block 300. The corresponding direction vectors would thenincline directly toward those selected points.

In addition to the steps illustrated in block 400 of FIG. 2A, the numberof radar pulses to be transmitted in order to conduct an effectiveFourier transform according to well-known, conventional SAR designprinciples, and thereby to obtain angle information regarding signalintensities of the radar return, must be determined conventionally.

Further, as expressly specified in block 400, a nominal pulse repetitioninterval (PRI) and /or pulse rate frequency is conventionallyestablished. Further, the nominal wavelength corresponding to thenominal carrier frequency is conventionally established.

Beyond this, the pulse timer is set to zero (t=0). Finally, forinitializing purposes the change in carrier frequency as represented bythe term "delta lambda" is set to zero.

In the case of three focal points involving the variation of both pulserate frequency and carrier frequency, constants "c₁ " and "c₂ " aredetermined as suggested at block 500, they respectively being equal to[a₁ ][nominal PRI] and [a₂ ][nominal PRI], where "a₁ "=(U₁ -U₀)(V) and"a₂ "=(U₂ -U₀)(V), where "V" is the antenna phase center velocityvector, and each respective vector quantity is representative of initialSAR conditions. Thus, c₁ =(U₁ -U₀)(V)(PRI) and c₂ =(U₂ -U₀)(V)(PRI).

Next, and before the transmission of a next radar pulse, as suggested atblock 600, the changed "delta t" and "delta lambda" values aredetermined, as follows: "delta t"=(c₁ b₂ -c₂ b₁)/D, where b₁ =(r₀-r₁)/lambda, and b₂ =(r₀ -r₂)/lambda; and "delta lambda"=(c₂ a₁ -c₁₂)/D, where a₁ =(U₁ -U₀)(V) and a₂ =(U₂ -U₀)(V) as before, and whereeach respective vector value is established as determined in theimmediately preceding step, and where D=a₁ b₂ -b₁ a₂. The velocity "V"is repeatedly determined by inertial sensors 119 mounted on the radarplatform 31, according to well known conventional techniques which arenot a part of this invention.

Upon accomplishing the above, the pulse timer 83 is reset as suggestedat block 700, by adding "delta t" to the previous timer setting, i.e.,t=t+"delta t". Further, the exciter/synthesizer 105 is adjusted tooperate at a frequency "f_(t) " corresponding to wavelength"lambda"="lambda"+"delta lambda", in view of the relationship "f_(t)"=c/lambda, where "c" is the speed of light if frequency is to bevaried.

Thereupon, the radar transmits a pulse at the next time according to theestablished carrier frequency "f_(t) ", receives a return signal andprocesses the return signal conventionally which established STALOfrequency "f_(t) -f_(x) " in mixer 39, and then processes the output ofthe mixer in conventional stages 39, 55, 59, 66, 77 and finally displaysthe results at map display 79.

The indicated process of system 13 then continues by returning forcontinued operation to repeat the steps indicated in block 600, untilthe number of radar pulses reaches the predetermined required number.The collected radar return information is in each case processedconventionally.

The above relationships are based upon coherent radar measurement ofdistance between the radar 13, or antenna phase center, and selectedpoints in the selected target area or on the field observed, using thewavelength lambda as a yardstick.

At any given time, the phase corresponding to a particular point "i" inthe target region being mapped is given by the expression: phi_(i)=(4pi)r_(i) /lambda, where "r_(i) " is the distance between the antennaphase center and the point "i", and "lambda" is the radar wavelength.

Selecting a central point "0", i.e. zero, for use as a point ofreference on the region to be mapped, the differential distance from theantenna phase center to the points "i" and "0" respectively will resultin the relationship:

    "phi.sub.i "=(4 pi)[r.sub.i -r.sub.0 ]/lambda,

where:

"r_(i) " is the distance to each of one or two selected points on theregion other than the map center; "r₀ " is the distance to the mapcenter from the radar platform; and "lambda" is the wavelength of theradar pulse to be transmitted.

To force each or either of the selected points "i" to be in fact a focalpoint, it is sufficient to ensure that the difference between theselected values of "phi_(i) " (e.g. "phi_(i) " -"phi_(o) ") vary by aconstant quantity for any two consecutive pulses. This will result inthe point "i" being represented by a delta function in the dopplerfrequency domain, corresponding to a focused condition.

This condition for a constant variation can be written as follows:"delta(phi_(i))", for each value of "i", equals the partial derivativeof "phi_(i) " with respect to time, multiplied by "delta t", plus thepartial derivative of "phi_(i) " with respect to wavelength multipliedby "delta lambda", which in turn equals c_(i), where the respective"c_(i) " values are constant values equal to the indicated sum ofpartial derivative factors, it being contemplated that there will be atleast two such points according to the invention; "phi_(i) " is thephase corresponding to a point "i" on the target; "delta t" is aselected time interval; "lambda" is the wavelength of radar pulsetransmitted; and "delta lambda" is the change in wavelength lambda ascan be implemented by one inventive version discussed herein.

After performing the partial derivative operation, the above expressionreduces to: "delta(phi_(i))"=(4 pi/lambda)[V(U_(i) -U₀) (delta t)-(r_(i)-r₀)("delta lambda"/lambda)], where: "delta (phi_(i))" is the phasechange in a signal traveling to point "i" and returning to the radarplatform; "V" is again the velocity vector representing the aircraftand/or radar platform velocity; "lambda" is the wavelength of thetransmitted pulse; "delta lambda" represents a change in the wavelengthor frequency made in the transmitted radar pulse with respect to aselected base wavelength or frequency; "U_(i) " is a unit vector fromthe antenna phase center to a selected point "i" in the region to bemapped; "U₀ " is the unit vector from the antenna phase center to themap center of the region to be mapped; and "r_(i) " is the distance fromthe antenna phase center to selected reference points including thecenter of the region to be mapped.

In the case of only two focal points, for evaluation under initialconditions, the map center, defined by distance "r₀ " and positionvector "U₀ ", and another selected point defined by "r₁ " and "U₁ "; thefollowing relationship results: "delta (phi)"=c₁ =[4 pi/lambda][V(U₁-U₀)(delta t)-(r₁ -r₀)(delta lambda/lambda)].

The above information is likely to induce those skilled in the art toconceive of variations of the invention which nonetheless fall withinthe scope thereof. Accordingly, reference to the claims which follow isurged as the claims alone specify with particularity the scope of thispatent.

I claim:
 1. An improved synthetic aperture radar system mounted on a moving platform, comprising:means for producing and receiving radar pulses, at a variable pulse repetition interval and at a carrier wavelength; means for processing selected received return radar pulses in a mapping operation to form a radar map of a target area containing a predetermined map reference point; means for performing a focus operation on said return radar pulses by applying a predetermined phase correction on selected return radar pulses to compensate for a change in platform position between detection of said selected return radar pulses, thereby bringing said map reference point in sharp focus; means for measuring the acceleration of said moving platform; and means for extending the focused area of said radar map to compensate for acceleration of said moving platform during the mapping process, wherein the improvement comprises: means for altering at least one of said variable pulse repetition interval and said carrier wavelength in a predetermined manner in response to signals from said means for measuring acceleration such that, for at least one point in said target area in addition to said map reference point, a phase difference between return radiation from said map reference point and return radiation from said at least one point is substantially constant for consecutive radar pulses, whereby both said map reference point and said at least one point are in sharp focus and said radar map has an improved depth of focus.
 2. The system of claim 1, further characterized in that said variable pulse repetition interval PRI is variable according to the relationship

    PRI=(c.sub.1)(λ/4π){V(U.sub.1 -U.sub.0)}

where "c₁ " is a system determined constant, "V" is the velocity of the radar platform, "U₁ " and "U₀ " are respective direction vectors from the moving platform to said two independent points in said target region to be held in focus, and "lambda" is the instantaneous carrier wavelength.
 3. The system of claim 1, further characterized in that said carrier wavelength is altered from a preceding carrier wavelength by an amount delta lambda given by the relationship

    delta lambda=/[(c.sub.2 lambda)-V(U.sub.1 -U.sub.0)PRI]lambda/(r.sub.0 -r.sub.1),

where c₂ is a system determined constant, V is the velocity of the radar platform, r₁ and r₀ are distances from said platform to said at least one point and said reference point and U₁ and U₀ are unit vectors directed from said platform toward said at least one point and said reference point.
 4. The system of claim 1, further characterized in that both said carrier wavelength and said variable pulse repetition interval are altered in a predetermined manner in response to signals from said means for measuring acceleration such that, for at least one point in said target area in addition to said map reference point, a phase difference between return radiation from said map reference point and return radiation from said at least one point is substantially constant for consecutive radar pulses, whereby both said map reference point and said at least one point are in sharp focus and said radar map has an improved depth of focus. 